An apparatus for generating x-ray radiation includes a cathode for generating an electron beam and an anode for decelerating the electrons of the electron beam and for generating an x-ray beam. Moreover, the apparatus includes a device for generating an electric field that is directed from the cathode in the direction of the anode.
In such an apparatus, the x-ray radiation arises from energetic transitions in the electron shells of atoms or molecules and from the change in velocity of the charged particles per se. In the apparatus, the electrons emitted by the cathode are initially accelerated by the applied electric field and are then incident on the anode, in which they are strongly decelerated. X-ray radiation and heat arise in the process, wherein electrons are ejected from the shells of the atoms as a result of electron and photon interactions. The holes in the shells are filled by other electrons, with, inter alia, the characteristic x-ray radiation arising. Overlaid thereon is the so-called bremsstrahlung, which is caused by the pure change in velocity of the electrons as a consequence of the interaction with the anode.
By way of example, x-ray radiation may be used to shine through the human body, with predominantly bones, but also internal organs, becoming visible. In the field of medical diagnostics, there is a desire to combine x-ray imaging with other imaging methods based on magnetic fields. By way of example, an apparatus for x-ray imaging may be combined with a magnetic resonance imaging (MRI) scanner. Magnetic fields may likewise arise for guiding the catheter in angiography, an imaging medical method which images blood and lymph vessels.
Medical apparatuses for generating x-rays may use hot cathodes. If hot cathodes are exposed to a strong magnetic induction, caused by a magnetic field device such as the MRI or the angiography system, the obtainable electron current is reduced. Likewise, the focusing of the electron beam emitted by the hot cathode is impaired by the optics that are characterized by electric fields. Hence, a substantially smaller electric current density (abbreviated to current density) arises at the anode in comparison with an x-ray apparatus without an external magnetic field. However, a certain, predetermined current density is required for generating the x-ray beam with an intensity that is sufficient for the medical application. It is possible to compensate the reduced current density by increasing the heating temperature of the hot cathode. However, such an increase in the heating temperature impairs the service life of the hot cathode and hence of the x-ray tube.